1. Field of the Invention
The present invention generally relates to an audio calibration system and more particularly to a technique for calibrating an audio system for any desired listening location. Still more particularly, the invention relates to a method and apparatus for tuning an audio system to cause a “null” point to be located at any desired location in the room.
2. Background of the Invention
Audio systems are designed to reproduce sound. Stereo systems, for example, reproduce sound from compact discs (CDs), cassette tapes, radio transmissions, and the like. Regardless of whether the audio system is a stand-alone stereo system, an audio system incorporated into a personal computer, or any other type of audio system, it is desirable to reproduce the sound as accurately as possible. Thus, audio systems are designed to recreate sound that is as close to the original recorded sound as possible.
Audio systems include a speaker, or other type of sound generating device, that converts an electrical signal into sound waves that emanate from the speaker, travel through the air, and into a person's ears. Audio systems include at least one speaker, and often include two or more speakers for stereo sound. A surround-sound system, for example, typically includes five speakers.
Sound takes a finite amount of time to travel between two points. The speed of sound through air at ground level is approximately one foot per millisecond. Thus, if a sound pulse emanates from two speakers at precisely the same instant in time, the sound pulse from both speakers will arrive at precisely the same time to a person that it is an equal distance from each speaker. If, however, that person is closer to one speaker than the other speaker, the sound pulse from the closer speaker or will arrive to the person before the samples from the other speaker, thereby causing a distortion in the perceived sound by the person. This problem is exacerbated in a surround-sound system in which the person listening to the system is located at different distances from each speaker.
One approach to solving this problem is to manually place each of the speakers connected to the audio system at an equal distance from where the listener is located. If the listener typically listens to music while sitting in a particular chair in a room, each of the speakers will be positioned the same distance from the listener's chair. However, if the listener wishes to listen to music from another position in the room, the speakers may have to be physically moved to calibrate the system to the new location. While the resulting sound quality is generally adequate, this technique often is difficult to implement because furniture in the room and other room specific factors may preclude conveniently locating speakers at equal distances from the listener's chair. Also, many people have speakers that are large enough to preclude being conveniently located in many locations in a room.
Another approach that has been suggested is to measure the distance between the listener's location and each speaker, calculate the difference between those distances, and introduce a time delay into the audio channel corresponding to the closer speaker. Thus, if the left speaker, in a two speaker system, is measured as being three feet closer to the listener than the right speaker, a technician programs a 3 millisecond time delay into the left audio channel. Three milliseconds is chosen in this example because at a speed of 1 foot per millisecond, it will take sound an extra 3 milliseconds to travel from the right speaker to the listener. The time delay in the left audio channel compensates for the difference in distance between the speakers and the listener. This technique usually requires a skilled technician to setup the audio system and thus, is expensive to perform and adjust if the listener wishes to change his or her listening location.
Thus, an improved technique for calibrating an audio system to a listener's location is needed. The new technique should be easy to perform and preferably not require a highly skilled technician. Despite the advantages such an audio calibrating system would offer, to date no such system is known to exist.